1. Field of the Invention
This invention relates to a dental dowel pin to be used in making crowns or bridges. More particularly, the invention relates to a dowel pin which is used as a core for a tooth model to facilitate removing and attaching of the tooth model with respect to a denture model in order to improve the making of the crown or bridge. That is, a tooth model-detachable model or a divisional replacing model which have been adopted to the technical working model by the indirect treatment.
2. Description of the Prior Art
Conventional dowel pins are metallic, and are in general composed of a bar-like portion to be implanted in the tooth model (primary plaster) and a pin body a major portion of which is supported in the denture model (secondary plaster). The pin body is frusto-conical and has one or both sides which are flattened by grinding. There are some pins equipped with needles on their heads.
Those existing dowel pins have difficulties as mentioned hereafter and improvement thereon is needed, more specifically.
(a) For implanting the dowel pin into hardened plaster, the adhesive is poured into an inserting hole prepared in advance therein. This is followed by insertion of the pin, but then the adhesive often overflows, as there is no escape for the adhesive and air from the blind pin hole. When putting water on the overflow adhesive, it turns up and spoils the precision of the model.
(b) The dowel pin is often implanted before the primary plaster becomes hardened, and in this case not only is it difficult to insert the pin into the tooth model at its center portion but also the pin often falls down due to the greater specific gravity of the metallic pin compared to that of the plaster. Therefore, to avoid falling, a dowel pin equipped with a needle is made to stand directly into a tooth impression surface as the primary plaster is poured. After removing the impression surface, the needle projects from the surface of the hardened plaster, i.e., the surface of the tooth model, this projection has to be removed. However, it is very difficult to undertake this removal without hurting the surface of the tooth model. In addition, dowel pins with such needles are expensive. There are other ways of controlling the needle falling, but those are unsatisfactory as to ease of use and reliability.
(c) The prior art dowel pins are metallic. The part acting as a female model to the dowel pin is the plaster, and there is a big difference in hardness between the metal and the plaster. During many repetitions of removal and attachment of the tooth model, the plaster's inner surface at the inserting hole is effectively ground which reduces the precision of the model, so that the final placing relationship of the tooth model to the denture model is out of order. Thus, the crown or bridge fitting the teeth precisely does not result. Further, if the dowel pin were often cut or ground by error while trimming, the interior of the inserting hole of the secondary plaster would be injured by the burr caused by the expansibility of the metal. In addition, metallic pins are susceptible to rusting by contact with the plaster.